The present invention relates generally to rotary mediums for recording thereon information signals and to apparatuses for recording the information signals thereon. More particularly, the invention relates to an information signal recording medium of rotary type on which pits for recording an information signal are formed without positively providing a groove for guiding a reproducing stylus and, moreover, in an arrangement such that a groove effect is not produced at the time of reproduction.
The systems known heretofore for recording and reproducing information signals such as video signals and/or audio signals on and from disc-shaped, rotary mediums (referred to hereinafter simply as discs) are broadly divided into systems in which reproduction is carried out optically, systems in which reproduction is carried out by means of a reproducing stylus employing a piezoelectric element, and systems in which reproduction is carried out by utilizing variations in the electrostatic capacitance between an electrode provided on the reproducing stylus and the recorded surface of the disc.
We have considered the advantages and disadvantages of these three kinds of systems and have adopted the electrostatic capacitance system as being the most desirable. Further, the present applicant has previously proposed, as a new system for overcoming the problems accompanying known electrostatic capacitance system, a novel "Information signal recording and reproducing system" as disclosed in the specification of U.S. Pat. Application Ser. No. 785,095.
More specifically, in accordance with a known electrostatic capacitance system, recording is accomplished in the recording system by forming a spiral guide groove in the disc for guiding the reproducing stylus and, at the same time, forming pits in responsive correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus, being guided by the guide groove, traces the track within the groove and thereby reproduces the signal in response to variations in the electrostatic capacitance.
In this known system however, since a reproducing stylus guiding groove is provided in the disc, and the reproducing stylus is compulsorily guided by this guide groove, it is not possible for the reproducing stylus to undergo operations such as riding over the groove wall of one track and moving into another track and returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out special reproduction such as quick-motion picture reproduction, slow-motion picture reproduction, and still-picture reproduction.
Furthermore, since the area of contact between the tip of the reproducing stylus and the guide groove of the disc is small, the stylus easily wears. When the width of the stylus tip reaches the groove width as a result of abrasive wear, the serviceable life of this reproducing stylus ends. Thus, this system is accompanied by the problem of short life of the reproducing stylus. Another problem is that fine chip particles abraded from the disc by the tracing action of the reproducing stylus along the guide groove give rise to difficulties such as further promotion of the wear and damage of the stylus and the guide groove and an increase of the gap between the electrode at the stylus tip and the recorded surface including the pits within the disc guide groove, whereby the reproduced output decreases because of spacing loss.
If the rotational speed of the disc is set at a low value such as 450 rpm., for example, in order to prolong the life of the stylus tip, the recording wavelength of the information signal recorded in the guide groove will become short. For this reason, a recording apparatus of low price in which laser light is used cannot be employed in the recording system, and the disadvantageous employing of a recording apparatus of high price using an electron beam becomes unavoidable.
Accordingly, with the aim of overcoming the various problems of the known systems described above, the present applicant has proposed by the aforecited patent applications a novel system for recording and reproducing information signals. According to this proposed system, in the recording system thereof, pits are formed in accordance with the information signal being recorded along a spiral track on a recording medium of flat disc shape, without forming a groove therein, and, in the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal. In this proposed system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal, and tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this previously proposed system, since the recording track has no groove, there is no possibility whatsoever of the reproducing stylus or the recording medium being damaged, and the stylus can trace the same portion of the track repeatedly a plurality of times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, other difficulties of the known system are removed.
Furthermore, as a reproducing stylus of highly desirable characteristics for reproducing an information signal recorded on a disc without a reproducing stylus guide groove, the present applicant has previously proposed a novel "Stylus for reproducing information signals recorded on a recording medium" as disclosed in the specification of U.S. Pat. Application Ser. No. 852,366 now U.S. Pat. No. 4,199,782. This proposed reproducing stylus has a bottom surface with a maximum width portion which is greater than the track pitch and has an electrode of a width smaller than the track pitch over a specific distance from the stylus tip.
A feature of this proposed reproducing stylus is that the stylus pressure per unit area is small, and the rate of abrasive wear of the reproducing stylus and the disc is low. Another feature of this stylus is that, since the electrode width does not increase appreciably even when the stylus tip becomes somewhat worn after a long period of use, the life of the reproducing stylus is remarkably long.
Thus, by using the recording and reproducing system and the reproducing stylus according to the above cited proposals, the problems encountered in the prior art are overcome, and very good results can be obtained.
Accordingly, the present applicant has been carrying out research on the reduction to practice of a system in which an information signal track is recorded on a smooth-surface disc without a groove with a track pitch greater than the track width (pit width), that is, with the information signal tracks in spaced-apart state, and a reference signal (hereinafter referred to as a pilot signal) for tracking control is recorded in the space between adjacent tracks, reproduction being carried out by the above mentioned reproducing stylus.
However, it has been found that, in spite of the absence from the beginning of a reproducing stylus guide groove in the disc and the fact that the reproducing stylus is contacting the disc over a width greater than the track pitch, certain adverse phenomena occur just as though a guide groove had been formed beforehand in the disc. More specifically, impairment of the aforementioned special modes of reproducing operation, decrease in the reproduced output due to spacing loss, or wear and damage of the reproducing stylus of the disc to a greater extent than expected occur. The cause of these phenomena has been determined to be the formation, in resultant effect, in the disc of a slight groove shape effect or a configuration effect which acts similarly as a groove.
Accordingly, the present inventors have studied this phenomenon of a groove guiding effect as a resultant effect and, as a result, have arrived at the following two causes thereof.
The first cause is that the track along which an information signal has been recorded is of very slight groove shape. Irrespective of whether the recording beam which forms the pits in the disc in response to the information signal being recorded is a light beam or whether it is an electron beam, its energy distribution over its cross section is not uniform. Furthermore, when the beam is projected onto the resist layer of the surface of the recording original disc, this beam is reflected and scattered at parts such as the outer surface, interior, and bottom surface of the resist layer. For these reasons, even when the beam is projected accurately onto the positions for forming the pits on the resist layer of the disc, a portion of the beam is projected also onto the surrounding peripheral parts of the intended positions for forming the pits. That is, undesired beam portions are projected onto also parts where, originally, the beam is not intended to be projected.
Consequently, those parts of the information signal recording track in the spaces between the pits are subjected more intensely than the parts of the spaces between adjacent tracks to the effect of the undesired beam portions. After the developing process, the recorded track parts are somewhat lower and depressed than the parts (land parts) of the spaces between adjacent tracks.
The second cause of the above described phenomenon is that the reproducing stylus is subjected to different forces respectively at the track parts and at the parts of the spaces between adjacent tracks. The pit density in the information signal track parts is high, and only the parts thereof of the spaces between the pits contact the reproducing stylus. Consequently, the area of contact of these parts is small, whereby these parts are easily recessed by the contact of the reproducing stylus, whereas the parts of the spaces between adjacent tracks are not easily recessed.
As a consequence, in the case where the disc surface is traced by a reproducing stylus having a bottom surface with a part of a width greater than the track pitch, the wear of the part of the bottom surface of the reproducing stylus contacting the track becomes small, while the wear of the part contacting the parts of the spaces between adjacent tracks becomes great because of the above described first and second causes. For this reason, the reproducing stylus is guided respectively by the track part and the parts of the spaces between adjacent tracks, whereby the above described phenomenon arises.
Accordingly, in the present invention, by a novel arrangement of the information track, a reproducing stylus guide groove is not intentionally provided, and, moreover, the occurrence of the effect of a reproducing stylus guide, even in resultant effect, is prevented.
On the other hand, the input-output characteristic of a reproducing apparatus for reproducing a rotary information signal recording medium, in general, is nonlinear, and the reproduced information signal and the reference signals are caused by this nonlinear characteristic to undergo cross modulation, giving rise to nonlinear distortion. For this reason, in the case where the information signal is a video signal, the picture quality of the reproduced picture deteriorates, while, in the case of an audio signal, the tone quality of the reproduced sound deteriorates.
One possible measure for preventing distortion (interference distortion) due to the above mentioned cross modulation is to effect an improvement of the linearity of the reproducing system. However, it is very difficult to cause the characteristics of the reproducing system to be perfectly linear, and a system so adapted would entail an extremely high cost.
The magnitude of the interference distortion occurring in the reproduced signal in the reproducing system corresponds to the difference between the levels of the information signal and the reference signals. Accordingly, by causing the level of the reproduced reference signal to be lower than the level of the reproduced information signals and, moreover, by causing the above level difference to be greater than a specific level difference, the magnitude of the interference distortion which develops can be reduced to a degree which is not detrimental for practical purposes.
However, in the case of reproduction of signals recorded by forming pits in the disc, the magnitude of the level of a reproduced output signal increases with the wavelength of signal recorded on the disc, increases with the pit depth up to a certain depth, and, furthermore, increases with the pit width (in the case where a reproducing stylus having an electrode width corresponding to the pit width is used). Therefore, by suitably selecting values such as the width and depth of the pits, the level of a reproduced output signal can be set at a desired value.